1. Field of the Invention
This invention relates to a process for forming embedded metallurgy patterns on a substrate, more particularly to a process for forming an interconnection metallurgy system embedded in the passivating layer with a planar top surface.
2. Description of the Prior Art
The forming of an interconnection metallurgy system for integrated circuit devices has conventionally been done by blanket depositing a metal layer, forming a photoresist layer on the metal layer, exposing the resist to the desired metallurgy pattern, developing the resist, and subsequently etching the exposed portions of the underlying metal layer to thereby form the interconnection metallurgy system. The pattern was subsequently covered by an insulating layer and another metallurgy pattern formed over same, making contact to the underlying layer through via holes, until the desired interconnection metallurgy system was complete. However, with continued miniaturization of semiconductor integrated circuits to achieve greater component density particularly in large scale integrated circuitry, the metallurgy was made smaller and more dense. The planarity of the surface of the system became a serious consideration in the fabrication of interconnection systems. Each time a metallurgy pattern is deposited on a surface, the more irregular or non-planar the surface of the overlying insulating layer becomes. In general, after three levels of metallurgy have been deposited, the surface becomes so irregular that additional layers cannot be deposited. The irregular surface presents two very important problems which have a direct bearing on the yield and reliability of the resultant system. When a layer of metal is deposited over an irregular surface, the resultant layer becomes thinner over a step portion of the supporting layer. This thinned down portion results in current crowding and possible failure due to electromigration. A further problem is concerned with forming the resist pattern. Clear, distinct exposure becomes impossible as the surface becomes more irregular.
With subtractive etching of the blanket layer of metal, the sidewalls of the resultant stripe are sloping because the etchant works downwardly as well as inwardly. This reduces the cross-section of the resultant stripe, limiting its current carrying capacity. A technique which was developed to overcome this problem was termed "expendable mask method" or "lift-off method" which was initially described and claimed in U.S. Pat. No. 2,559,389. Improvements to the basic lift-off method have been made, as for example in commonly assigned U.S. Pat. Nos. 3,849,136 filed Aug. 31, 1973 and 3,873,361 filed Nov. 29, 1973. However, the lift-off technique for forming a metallurgy system does not overcome the non-planarity problem discussed previously.